Encoders provide a measurement of the position of a component in a system relative to some predetermined reference point. Encoders are typically used to provide a closed-loop feedback system to a motor or other actuator. For example, a shaft encoder outputs a digital signal that indicates the position of the rotating shaft relative to some known reference position that is not moving. A linear encoder measures the distance between the present position of a moveable carriage and a reference position that is fixed with respect to the moveable carriage as the moveable carriage moves along a predetermined path.
Optical encoders utilize a light source and a photodetector to measure changes in the relative position of an encoding disk or strip. In a transmissive encoder, the encoding strip includes a series of alternating opaque and transparent bands. The light source is located on one side of the encoding strip, and the photodetector is located on the other side of the encoding disk. The light source and the photodetector are fixed relative to one another, and the encoding strip moves between the light source and the photodetector such that the light reaching the photodetector is interrupted by the opaque regions of the encoding strip. The position of the encoding strip is determined by measuring the transitions between the light and dark regions observed by the photodetector.
In a reflective encoder, the light source and photodetector are located on the same side of the encoding strip, and the encoding pattern consists of alternating reflective and absorbing bands. The light source is positioned such that light from the light source is reflected onto the photodetector when the light is reflected from the reflective bands.
Transmissive encoders have a number of advantages over reflective encoders in terms of tolerance. In a transmissive encoder, the light from the light source is collimated before it reaches the encoding strip, and hence, the light leaving the encoding strip is also collimated. The detection assembly needs only to image this collimated light onto the detector surface. Hence, the only critical distance is that between the imaging lens and the detector. This distance remains constant even if the distance between the code strip and the detector varies during the relative movement of the code strip and the detector.
In a reflective encoder, the distance between the code strip and the detector is critical as either the encoding strip itself or the light source as seen in the reflected light from the encoding strip is imaged into the detector. Hence, if there is an error in the code strip to detector module distance, the image will be out of focus and errors will result.
Unfortunately, transmissive encoders require that the two separate components, the light source and photodetector, be mounted and aligned with one another at the time of assembly of the encoder. This increases the burden on the manufacturer of the final product that incorporates the encoder. Reflective encoders, in contrast, are constructed from a single emitter-detector element that is packaged together with the various optical components for imaging the light source onto the photodetector. Hence, the manufacturer only has to mount and align one component. This reduces the cost of assembly from the manufacturer's point of view. In addition, in many applications involving miniature motors and the like, providing mounting locations on both sides of the code strip poses problems. Hence, if the problems associated with maintaining the correct distance between the code strip and the emitter detector module could be overcome, reflective encoders would be preferred.